1. Field of the Invention
The present invention relates to a liquid ejecting head for ejecting a liquid drop from a nozzle opening by changing a pressure of a liquid inside a pressure chamber interconnected to the nozzle opening, and a liquid ejecting apparatus having the liquid ejecting head.
2. Description of the Related Art
As a representative example of conventional liquid ejecting apparatuses, there is an ink jet recording apparatus having an ink jet recording head for image recording. As other liquid ejecting apparatuses, for example, an apparatus having a coloring material ejecting head used to manufacture a color filter for a liquid crystal display, an apparatus having an electrode material (conductive paste) ejecting head used to form an electrode for an organic EL display or a face emission display (FED), an apparatus having a biological organic substance ejecting head used to manufacture biological chips, and an apparatus having a sample ejecting head as a precise pipette may be cited.
The ink jet recording apparatus, which is a representative example of the liquid ejecting apparatus, has been used in various types of printing including color printing in recent years since noise during printing is comparatively low and moreover small dots can be formed at a high density.
Such an ink jet recording apparatus generally includes an ink jet recording head moving back and forth in the width direction (head scanning direction) of a recording medium (a processed article) such as a recording form loaded on a carriage and a paper feed mechanism for moving the recording medium in the direction (feeding direction) perpendicular to the head scanning direction and additionally includes a platen arranged opposite to the recording head for supporting the recording medium fed by the paper feed mechanism from the rear thereof and defining the position of the recording medium for the recording head.
In this ink jet recording apparatus, printing is executed by ejecting ink drops from the recording head to the recording medium in correspondence to print data. And, the recording head loaded on the carriage is designed so as to eject ink of various colors, for example, black, yellow, cyan, and magenta, thus not only text printing by black ink but also full-color printing by changing the ejection rate of each ink can be executed.
When printing the overall surface of a recording medium without leaving blanks on the edges thereof (so-called four-side edge-free printing) using this ink jet recording apparatus, in consideration of an allowance for shifting of the recording medium and carriage, an area slightly wider than the size of the recording medium is printed.
Namely, to print the recording medium without leaving blanks on the left and right edges (the edges extending in the feeding direction) thereof, the scanning range of the recording head during printing can be set to positions shifted outside from the edges of the recording medium.
Furthermore, when printing the recording medium without leaving blanks on the front and rear edges (the edges extending in the head scanning direction) thereof, at start time of printing of the recording medium, up to an area shifted forward from the front edge of the recording medium is designated as an area to be printed and at end time of printing of the recording medium, up to an area shifted backward from the rear edge of the recording medium is designated as an area to be printed.
And, ink drops ejected toward the areas other than the recording medium are absorbed by an absorption member (sponge, etc.) arranged on the rear side of the recording medium opposite to the recording head.
As mentioned above, when printing the recording medium without leaving blanks on the edges thereof, ink drops are ejected into the areas shifted from the edges thereof in the longitudinal direction or transverse direction. Therefore, a problem arises that an ink mist moved on the rear side of the recording medium is stuck to the edges of the rear thereof, thus the recording medium is soiled. Particularly, a case of executing double-side printing on a recording medium and a case of printing a recording medium using both sides like a postal card cause a problem. Further, problems also arise that misted ink soils the inside of the apparatus, and an operation defect is caused by adhesion of an ink mist to the electrical circuit and linear scale, and an ink mist is accumulated on the ink cartridge, thereby soils the user's hands.
Further, generally, the feed mechanism for feeding a recording medium in the feeding direction has rollers arranged opposite to each other so as to hold and feed it. One of the rollers arranged opposite to each other is a drive roller having a structure of baking aluminum on the surface of a metallic roller to improve the frictional force and the other roller is a follower roller made of plastics.
And, generally, the recording medium, at the point of time when it is fed into the print area, is electrically charged by contact or separation between the rollers and the recording medium, rubbing with the next recording medium when feeding the recording medium from the auto sheet feeder, or contact with the structural member on the feed route of the recording medium. And, when the recording medium is electrically charged like this, an ink mist is easily stuck to the rear of the recording medium.
To solve those problems, for example, a method for generating an electric field between a nozzle plate and an absorption member so as to move ink drops up to the absorption member by the Coulomb force, thereby preventing them from misting is proposed. This method is required to generate an electric field so as to inject an electrical charge into ink drops. Conventionally, the nozzle plate is formed by a metallic member (conductive member), so that an electrical charge can be injected into ink via the contact surface between the nozzle plate and the ink. In the metallic nozzle plate, the nozzle openings are formed by pressing.
As a document disclosing the related art, for example, JP-A No. 2004-202867 may be referred to.
However, in order to realize a higher image quality and a higher speed of the ink jet recording apparatus, it is necessary to increase the density of a plurality of nozzle openings (nozzle density) formed on the recording head, thus it is difficult to form nozzles by pressing of a metallic plate.
As a method for forming nozzle openings at a high density, laser processing for a resin plate using, for example, excimer laser, photolithography using photosensitive resin, and dry etching for an Si substrate are known.
However, nozzle plates manufactured by these methods are all formed by a non-conductive member, so that it is impossible to inject an electrical charge into ink drops via a nozzle plate.
When ink in the recording head is not electrically conducted to the ground (Gnd) (or to the power source), ink drops can be electrically charged initially by dielectric polarization. However, the ink in the recording head is gradually charged and becomes equal to the potential on the absorption member side, thus no potential difference is generated so that the effect cannot be continued.
Further, when the electrode for injecting an electrical charge into ink is installed in the ink flow path of the recording head, an electric field is generated in the ink in the recording head. Therefore, there are worries that electrophoresis occurs in the ink in the recording head, thus the image quality is deteriorated due to irregularities in the coloring material density in the recording head or electrolysis occurs in aqueous ink, and foreign substances and gas are generated, thus defective ejection is caused.
Further, there is a nozzle plate formed by electroforming using Ni, etc. A problem arises that electroforming is not productive and is expensive.
Further, conventionally, for the flow path substrate in which the pressure chambers interconnected to the nozzle openings and the flow path for feeding ink to the pressure chambers are formed, a Si single-crystal substrate is used in order to form a precise flow path at a high aspect ratio. When the recording head becomes longer in correspondence with an increase in the printing speed of the ink jet recording apparatus, if the flow path substrate and nozzle plate are formed by materials different in the coefficient of linear expansion, it is difficult to ensure the reliability of the head with respect to change in the temperature environment.
Also from this viewpoint, in place of the conventional metallic nozzle plate, it is preferable to use a nozzle plate of the same material as that of the flow path substrate. In this case, the nozzle plate is non-conductive so that the aforementioned problem arises with respect to the injection of an electrical charge into ink drops.